1. Technical Field
This invention relates to semiconductor processing. More specifically the invention relates to photo acid generator (PAG) compounds, photo resists using PAGs, and a method for improving nested to isolated line bias using such photo resists.
2. Background Art
Lithographic processes are used in the manufacture of semiconductor devices, such as integrated circuit chips and read/write heads for magnetic media drives, e.g. disc drives and tape drives. Lithographic processes typically involve a step of depositing a layer of a photo resist material onto a device such as a silicon wafer by, e.g. spin-coating. The next step typically involves baking the device at a temperature of approximately 110.degree. C. (Celsius), although high bakes can occur at temperatures above 140.degree. C., to set the photo resist layer. Next comes selectively exposing portions of the photo resist layer to radiation, e.g. ultraviolet (UV) light, or other ionizing radiation, e.g. X-ray or ion or electron beam. The last step includes developing the photo resist layer by washing with a basic developer solution, e.g. tetramethylammonium hydroxide (TMAH), thereby removing the non-irradiated portions of a negative photo resist or the irradiated portions a positive photo resist. Lithographic processes, along with typical photo resist materials, are described by W. Moreau, Semiconductor Lithography (1989), which is incorporated herein by reference for its pertinent and supportive teachings.
One type of photo resist, a chemically amplified photo resist (CAMP), uses acid catalysis. CAMP is formulated by dissolving an acid sensitive polymer and a photo acid generator (PAG) in a casting solvent. CAMP is particularly useful when relatively short wavelength radiation is used, including deep UV radiation, e.g. 190-315 nanometer (nm) wavelength, and mid UV radiation, e.g. 350-450 nm wavelength. The shorter wavelengths are typically desired to increase resolution and, thus, decrease the feature size of semiconductor devices, but fewer photons are radiated for a given energy dose. Accordingly, higher exposure doses are typically required when using UV radiation to obtain a sufficient photochemical response in the resist unless CAMP is used. Generally, non-amplified resists require a dose of greater than 200 millijoules per square centimeter (mJ/cm.sup.2) while doses of less than 500 mJ/cm.sup.2 may suffice for CAMPs.
In a CAMP, acid sensitivity of the polymer exists because acid sensitive side chain groups are bonded to the polymer backbone. When exposed to radiation, the PAG produces acid that, when the resist is heated, causes catalytic cleavage of the acid sensitive side chain groups. A single acid catalyst molecule generated in this manner may be capable of cleaving multiple side chain groups, thus allowing lower exposure doses for the needed photochemical response. In a typical positive resist, the exposed polymer is then soluble in an aqueous base developer while, in a negative resist, the unexposed polymer is then soluble.
Several acid catalyzed CAMPs have been developed, although their PAGs are primarily best suited for deep UV radiation, typically at about 248 nm. By comparison, there are very few, if any, effective PAGs well suited for mid UV radiation, typically I-line radiation at about 365 nm. A standard mid UV CAMP uses a deep UV PAG along with a photosensitizing additive, commonly called a sensitizer, to adapt the deep UV PAG for mid UV radiation. Such sensitizers are also called absorbers or initiators, indicative of their function as they absorb I-line radiation or chemically initiate the formation of acid by the deep UV PAG. Without sensitizers, a deep UV PAG does not sufficiently absorb mid UV radiation to achieve the potential photochemical response.
Unfortunately, there are several problems associated with the use of sensitizers in converting a deep UV PAG to a mid UV PAG. First, existing sensitizers are not highly soluble in photo resist casting solvents or developer solutions, and consequently, the concentration of sensitizer that can be employed in the photo resist formulation is limited.
Second, existing sensitizers are susceptible to vaporization, e.g. by sublimation, during the baking process, thereby depleting the photo resist formulation of sensitizer. The vaporized sensitizer can also coat the baking tools and then impair processing when it subsequently flakes off. Third, many existing sensitizers lack sufficient thermal stability to be of practical use since they decompose at the elevated baking temperatures, e.g. above 110.degree. C., often needed in photo resist processing. Fourth, conventional deep UV PAG/I-line sensitizer combinations lack adequate photo speed. That is, they typically require relatively long exposure times to reach the proper exposure level, and accordingly increase processing time and expense. Fifth, many available deep UV PAG/I-line sensitizer combinations yield a relatively poor nested to isolated line bias. That is, the line width variation is too high when comparing a group of nested lines to an isolated line, where all lines were designed to have the same width.
Several PAGs, sensitizers, and PAG/sensitizer combinations have been proposed to remedy some of the problems indicated above for deep UV and mid UV CAMPs. While each composition addresses one or a few of the listed problems, none adequately address all of the problems. One such composition is described in U.S. patent application Ser. No. 08/672,804, entitled "POLYMER-BOUND SENSITIZER", filed Jun. 28, 1996 by Jagannathan, et al., and assigned to International Business Machines Corporation, Armonk, N.Y. Other compositions are described in U.S. Pat. No. 5,585,220 to Breyta et al., U.S. Pat. No. 5,492,793 to Breyta et al., U.S. Pat. No. 5,368,993 to Thackeray et al., U.S. Pat. No. 5,362,599 to Knors et al., U.S. Pat. No. 5,340,696 to Thackeray et al., U.S. Pat. No. 5,332,650 to Murata et al., U.S. Pat. No. 5,296,332 to Sachdev et al., U.S. Pat. No. 4,810,613 to Osuch et al., U.S. Pat. No. 4,760,013 to Hacker et al., U.S. Pat. No. 4,491,628 to Ito et al., and U.S. Pat. No. 4,371,605 to Renner. Each of the above listed references is herein incorporated by reference for their pertinent and supportive teachings.
Thus, it can be seen from the above discussion that it would be an improvement in the art to provide a thermally stable mid UV PAG that increases photo speed above currently available mid UV PAGs and improves nested to isolated line bias, yet, adequately dissolves in casting solvents and developer solutions and adequately resists vaporization.